MP8772

– 17V, 12A, SYNCHRONOUS, STEP-DOWN CONVERTER

MP8772 Rev. 1.0

www.MonolithicPower.com

14

1/15/2018

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OPERATION

The MP8772 is a fully integrated, synchronous,

rectified, step-down, switch-mode converter.

Constant-on-time (COT) control is employed to

provide fast transient response and ease loop

stabilization. Figure 2 shows the simplified ramp

compensation block in the MP8772. At the

beginning of each cycle, the high-side MOSFET

(HS-FET) is turned on when the feedback

voltage. The on period is determined by both the

output voltage and input voltage to make the

switching frequency fairly constant over the input

voltage range.

After the on period elapses, the HS-FET is

turned off. The HS-FET is turned on again when

this way, the converter regulates the output

voltage. The integrated low-side MOSFET (LS-

FET) is turned on when the HS-FET is in its off

state to minimize conduction loss. There is a

dead short between the input and GND if both

the HS-FET and LS-FET are turned on at the

same time. This is called shoot-through. To

avoid shoot-through, a dead time (DT) is

generated internally between the HS-FET off

and LS-FET on period or the LS-FET off and HS-

FET on period.

Internal compensation is applied for COT control

to provide a more stable operation, even when

ceramic

capacitors

are

used

as

output

capacitors. This internal compensation improves

jitter performance without affecting the line or

load regulation.

VOUT

L

Cout

R1

R2

On

Timer

Logic

Control

REF

SW

RAMP

FB

RAMP

GENERATOR

PWM

BUF

Figure 2: Simplified Ramp Compensation Block

Heavy-Load Operation

Continuous conduction mode (CCM) is when the

output current is high and the inductor current is

always above zero amps (see Figure 3). When

interval determined by the one-shot on-timer.

When the HS-FET is turned off, the LS-FET is

turned on until the next period.

Figure 3: Heavy-Load Operation

In CCM operation, the switching frequency is

fairly

constant.

This

is

called

pulse-width

modulation (PWM) mode.

Light-Load Operation

When the MP8772 works in pulse-frequency

modulation (PFM) during light-load operation,

the MP8772 reduces the switching frequency

automatically to maintain high efficiency, and the

inductor current drops almost to zero. When the

inductor current reaches zero, the LS-FET driver

goes

into

tri-state

(Hi-Z)

(see

Figure

4).

Therefore, the output capacitors discharge

slowly to GND through the LS-FET, R1, and R2.

This operation improves device efficiency greatly

when the output current is low.

Ton is constant

Figure 4: Light-Load Operation